Lithium ion capacitors and methods of production

ABSTRACT

A lithium-ion capacitor may include a cathode, an anode, a separator disposed between the cathode and the anode, a lithium composite material, and an electrolyte solution. The cathode and anode may be non-porous. The lithium composite material comprises a core of lithium metal and a coating of a complex lithium salt that encapsulates the core. In use, the complex lithium salt may dissolve into and constitute a portion of the electrolyte solution.

This application is a continuation of U.S. patent application Ser. No.13/687,161 filed on Nov. 28, 2012, the content of which is relied uponand incorporated herein by reference in its entirety, and the benefit ofpriority under 35 U.S.C. §120 is hereby claimed.

BACKGROUND

1. Field

The present disclosure relates generally to electrochemical energystorage devices, and more specifically to lithium-ion capacitors andtheir methods of production.

2. Technical Background

Capacitors, including double layer capacitors, have been utilized inmany electrical applications where a pulse of power is required. Somelithium-ion capacitors may have a significantly higher power densitythan standard ultracapacitors. However, many ultracapacitors have arelatively low energy density for selected purposes.

Lithium-ion capacitors contain a faradaic electrode (anode) and anactivated carbon electrode (cathode) where there are no faradaicreactions. These capacitors have advantages associated with a battery(with respect to their high energy density) and a capacitor (withrespect to their high power capability). For instance, lithium-ioncapacitors can provide higher operating voltage (˜3.8-4V) compared to anEDLC device voltage of 2.5 to 2.7V.

Lithium-ion capacitors have been proposed to address the insufficientenergy density in ultracapacitors and other standard capacitors. Forlithium-ion based capacitors, currently-proposed models require that alithium metal electrode, in addition to a cathode and an anode, beincorporated into the device. The result is an electrochemical energystorage device with three electrodes (cathode, anode, and lithium metalelectrode).

Such three electrode devices require the use of a porous cathode inconjunction with a mesh-type current collector in order to facilitatetransport of lithium into and within the cell. The fabrication of porouselectrodes and the construction of the overall three-electrode capacitordesign can be complicated, and such a cell may be expensive tomanufacture. Additionally, the presence of a lithium metal electrode inthe capacitor presents design challenges, as lithium metal ispotentially combustible in the presence of air.

BRIEF SUMMARY

In accordance with embodiments of the present disclosure, a lithium-ioncapacitor comprises a non-porous cathode, a non-porous anode, aseparator positioned between the cathode and the anode, a lithiumcomposite material positioned between the anode and the separator, andan electrolyte solution. The electrolyte solution comprises anelectrolyte material (solute) dissolved in a solvent. The lithiumcomposite material comprises a lithium metal core and a layer of acomplex lithium salt encapsulating the core.

In the assembled structure, the solvent may dissolve the complex lithiumsalt such that the electrolyte material comprises or consistsessentially of the complex lithium salt. The lithium composite materialmay be a source of both electrolyte material and lithium for thecapacitor, e.g., a sole source of lithium.

Following dissolution of the lithium composite material, in a furtherembodiment, a lithium-ion capacitor comprises a non-porous cathode, anon-porous anode, a separator positioned between the cathode and theanode, and an electrolyte solution.

Additional features and advantages of the subject matter of the presentdisclosure will be set forth in the detailed description which follows,and in part will be readily apparent to those skilled in the art fromthat description or recognized by practicing the subject matter of thepresent disclosure as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the subjectmatter of the present disclosure, and are intended to provide anoverview or framework for understanding the nature and character of thesubject matter of the present disclosure as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe subject matter of the present disclosure, and are incorporated intoand constitute a part of this specification. The drawings illustratevarious embodiments of the subject matter of the present disclosure andtogether with the description serve to explain the principles andoperations of the subject matter of the present disclosure.Additionally, the drawings and descriptions are meant to be merelyillustrative, and are not intended to limit the scope of the claims inany manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of specific embodiments of thepresent disclosure can be best understood when read in conjunction withthe following drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 is a schematic diagram of a lithium-ion capacitor according toembodiments;

FIG. 2 is a cross-sectional view of a lithium composite particle;

FIG. 3 is a plot of constant-current discharge curves for examplelithium-ion capacitors according to one embodiment;

FIG. 4 is a series of Ragone plots for example lithium-ion capacitors;

FIG. 5 is a series of constant-current discharge curves for examplelithium-ion capacitors according to a further embodiment;

FIG. 6 shows cyclic voltammograms of example lithium-ion capacitors;

FIGS. 7A and 7B are SEM micrographs of LiPF₆-coated lithium metalparticles according to one embodiment; and

FIG. 8 is an SEM micrograph of LiPF₆-coated lithium metal particlesaccording to a further embodiment.

DETAILED DESCRIPTION

Reference will now be made in greater detail to various embodiments ofthe subject matter of the present disclosure, some embodiments of whichare illustrated in the accompanying drawings. Whenever possible, thesame reference numerals will be used throughout the drawings to refer tothe same or similar parts.

The lithium-ion capacitors disclosed herein may comprise a non-porouscathode, a non-porous anode, a separator positioned between the cathodeand the anode, a lithium composite material positioned between the anodeand the separator, and an electrolyte solution. The electrolyte solutionincludes an electrolyte material (solute) dissolved in a solvent. Thelithium composite material comprises a lithium metal core and a layer ofa complex lithium salt encapsulating the core.

In one embodiment, the lithium composite material may include aplurality of stabilized lithium composite particles each having a coreand a coating material of a complex lithium salt surrounding andencapsulating the core. Incorporation of the lithium composite materialinto the capacitor eliminates the need for a separate lithium metalelectrode. In some embodiments, the electrochemical performance of thecapacitor may be improved due to the lack of a lithium metal electrodeand the attendant volume and weight savings for the overall cell.

In constructing the lithium-ion capacitor, a lithium composite materialmay be used as a source of at least a portion of the electrolytematerial and at least a portion of the lithium metal used by the cell.For example, the complex lithium salt that encapsulates the lithiummetal core of the composite particles may dissolve in the electrolytesolvent of an assembled cell and constitute a portion, or substantiallyall, of the electrolyte material in the electrolyte solution. These aswell as other benefits of the lithium-ion capacitors of the currentdisclosure are described in detail herein.

Referring now to FIG. 1, a lithium-ion capacitor 100 according to oneembodiment comprises a cathode 120, an anode 110, and a separator 150 ina stacked configuration. The cathode 120 may comprise an outer surface124 and a separator-facing surface 122, and the anode 110 may comprisean outer surface 114 and a separator-facing surface 112. As illustrated,the separator 150 is positioned between the cathode 120 and the anode110, such that the separator comprises a cathode-facing surface 152 andan anode facing surface 154.

The separator 150 may be a lithium ion-permeable membrane configured tobe positioned between the cathode 120 and anode 110 that keeps the anodeand cathode from contacting each other.

The cathode 120 includes a cathode material that may comprise activatedcarbon, or any other suitable cathode material for a lithium-ioncapacitor. As used herein, an activated carbon material has a specificsurface area greater than about 500 m²/g.

The anode 110 includes an anode material that may comprise graphite,carbon black, hard carbon, coke, or combinations thereof. Hard carbonmaterial, as used herein, has a specific surface area less than about500 m²/g. In some embodiments, the cathode 120 and anode 110 may benon-porous, and may be impermeable to liquids including solvents used toform an electrolyte solution. The cathode 120 and anode 110 may beattached to respective positive and negative current collectors (notshown).

The cathode 120, anode 110 and current collectors when assembled maycollectively be referred to as an electrode set. In conventionallithium-ion capacitors, the electrode set may further comprise a lithiummetal electrode. According to the present disclosure, lithium-ioncapacitor 100 does not contain a lithium metal electrode. In someembodiments, the electrode set may consist essentially of cathode 120and anode 110, or consist essentially of a cathode 120, anode 110, andrespective current collectors.

A liquid electrolyte solution 170 may be incorporated between thecathode 120 and anode 110 such that the electrolyte solution permeatesthe separator 150. The electrolyte solution 170 may comprise anelectrolyte material (solute) dissolved in a suitable solvent. Theelectrolyte material may be any material capable of functioning in anelectrochemical device. In embodiments, the electrolyte material may bea lithium salt, i.e., a complex lithium salt such as LiPF₆, LiBF₄,LiClO₄, LiAsF₆, or LiF₃SO₃, as well as mixtures thereof. Examplesolvents for forming an electrolyte solution include organic solvents ormixtures of organic solvents such as dimethyl carbonate, methylpropionate, ethylene carbonate, propylene carbonate, diethyl carbonateas well as other solvents suitable for use in an electrolyte where thelithium-ion is the charge carrier. In some embodiments, the solvent maybe capable of dissolving the electrolyte material of the lithiumcomposite material.

A complex lithium salt is any ionic compound comprising lithium and anadditional metal, metalloid or non-metal atom that does not itselfionize and which is soluble in an organic solvent. For instance, LiPF₆contains lithium and phosphorus as metal atoms, but the phosphorus doesnot ionize by itself. Rather, phosphorus ionizes as the PF₆ ⁻ ion. In afurther example, LiBF₄ contains lithium metal and the metalloid boron.Although lithium-ionizes (Li⁺), boron does not ionize by itself, but asthe BF₄ ⁻ ion. In a still further example, LiClO₄ contains lithium metaland the non-metal atoms chlorine and oxygen. The non-metal atoms ionizeas the perchlorate ion (ClO₄ ⁻). The solvent may be any suitable solventfor use in an electrochemical energy storage device.

The composite lithium particles 160 may be positioned between thecathode 120 and the anode 110. In the illustrated embodiment, thecomposite lithium particles 160 are disposed on a separator-facingsurface 112 of anode 110. In a related embodiment, the composite lithiumparticles 160 are disposed on anode facing surface 154 of separator 150.The composite lithium particles may be incorporated into the device as acontiguous layer on or both of a surface of the anode or a surface ofthe separator.

The amount of composite lithium particles incorporated into the devicemay be chosen to provide the desired amount of lithium metal (from thecore of the composite particles), the desired amount of electrolytematerial (from the complex lithium salt layer that encapsulates thecore), or both.

A weight ratio of lithium metal to anode (e.g., lithium:graphite) may bein the range of about 1:2 to 1:10, e.g., 1:2, 1:3, 1:4, 1:5, 1:6, 1:7,1:8, 1:9 or 1:10, or any range between any two ratios disclosed. In oneembodiment, the weight ratio of lithium metal to anode (e.g., graphite)may be in the range of about 1:3 to 1:6.

An as-assembled lithium-ion capacitor according to embodiments includesan anode, a cathode, a separator disposed between the anode and thecathode, and lithium composite particles disposed between the anode andthe separator. Upon addition of a liquid electrolyte (or electrolytesolvent) to the system, the lithium salt that coats the lithiumcomposite particles may dissolve into and thus form a component of theelectrolyte solution. The electrolyte solvent may be selected andprovided in an amount sufficient to contact and dissolve the complexsome or substantially all of the lithium salt of the lithium compositematerial 160.

One method of forming a lithium-ion capacitor comprises assembling anelectrode set comprising an anode, a cathode, a separator disposedbetween the anode and the cathode, and lithium composite particlesdisposed between the anode and the separator and then adding anelectrolyte solution to the assembly. A further method of forming alithium-ion capacitor comprises assembling the foregoing electrode setand adding an electrolyte solvent to the assembly.

During use, i.e., as a consequence of charging and discharging the cell,the complex lithium salt coating on the lithium composite particles maycompletely dissolve into and form a component of the electrolytesolution.

The lithium composite particles described herein generally comprise acore and a coating that encapsulates the core. The core may compriselithium metal or a lithium metal alloy. The coating, which comprises alithium salt, surrounds and encapsulates the core. The coating may behermetic and thus prevent or substantially inhibit water or air,including oxygen, from contacting and reacting with the core. Astabilized lithium composite material may be substantially non-reactiveor non-combustive if exposed to air, oxygen or water, such as an ambientenvironment. Thus, in embodiments the composite particles are stabilizedwith respect to ambient exposure.

A single, stabilized lithium composite particle 300 is shownschematically in cross-section in FIG. 2. Particle 300 includes a core310 and a coating 320 that completely surrounds and encapsulates thecore. The core 310 may comprises a unitary body defining an outersurface 312. The coating 320 is in direct physical contact with theouter surface 312 of the core 310 along an inner surface 324 of thecoating 320. The coating is inorganic and is free of organic speciessuch as mineral oil.

The core 310 in some embodiments comprises lithium metal, sometimesreferred to as elemental lithium. In further embodiments, the core maycomprise an alloy of lithium. Examples of such alloys comprise lithiumand one or more of Al, Si, Ge, Sn, Pb and Bi. The coating 320 comprisesa lithium salt that may include a complex lithium salt such as LiPF₆,LiBF₄, LiClO₄, LiAsF₆, or LiF₃SO₃, as well as mixtures thereof. Such asalt is soluble in standard organic solvents, including dimethylcarbonate, methyl propionate, ethylene carbonate, propylene carbonate,and diethyl carbonate.

As illustrated in FIG. 2, the core 310 has a particle size 336, and thestabilized lithium particle 300 has a particle size 334. The term“particle size” is used to describe the maximum linear dimensionassociated with a particle. In the case of a spherical particle, forexample, the particle size is the diameter. In the case of an oblongparticle, the particle size is the “length” of the particle. An exampleaverage particle size for a plurality of composite particles 300 mayrange from about 5 microns to 500 microns, e.g., 5, 10, 20, 50, 300,150, 200, 300, 400 or 500 microns, and may be defined for a givenmaterial batch over a range of any two of the aforementioned values.

The coating 320 has a thickness 332 defined as the average shortestdistance between the inner surface 324 of the coating and the outersurface 322 of the coating. In embodiments, the coating may have asubstantially uniform thickness or a variable thickness depending, forexample, on the method used to form the coating. An example averagethickness for the coating 324 may range from about 10 nm to 300 microns,e.g., 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50 or 300microns, which may be defined for a given material batch over a range ofany two of the aforementioned values.

In some embodiments, the stabilized lithium composite particle 300 maybe substantially spherically shaped. However, other shapes arecontemplated herein, such as, but not limited to asymmetric shapes orspheroids.

The stabilized lithium composite particles 300 are substantiallynon-reactive or non-combustive if exposed to air, oxygen or water. Thecoating 320 encapsulates the lithium core 310 to substantially inhibitor prevent exposure and reaction of the lithium with ambient gases orliquids. The stabilized lithium composite particles 300 may besubstantially chemically inert, for example, to ambient exposure or toelevated temperature (e.g., 50° C., 300° C., 150° C. or even 200° C.)exposure to air, oxygen or water vapor. The stabilized lithium compositeparticles may be sufficiently stable to be stored in air for at leastone week, 2 weeks, 1 month, or even a year without substantial chemicaldegradation and/or combustion.

Stabilized lithium composite particles may be produced by providinglithium metal particles, and contacting the lithium metal particles witha coating solution that includes a coating material dissolved in asolvent. The coating material may include a lithium salt or complexlithium salt as described above. The contacting may be performed byimmersing the particles in the solution or by other means such as spraycoating. After coating the particles, the solvent is removed to form alayer of the coating material over the lithium metal particles. Removalof the coating solvent may be performed by evaporation.

Due to its high reactivity and flammability, lithium metal is oftenstored under the cover of a viscous hydrocarbon such as mineral oil.While the mineral oil encapsulant inhibits degradation of the lithiummetal, it is generally incompatible with most solid state devices. Withthe present stabilization approach, the lithium particles are safe tohandle and store, and can be incorporated into a lithium-ion devicedirectly in their stabilized form.

In an embodiment, stabilized lithium composite particles may be producedby initially providing lithium metal or lithium metal-containingparticles that are immersed in mineral oil. Prior to forming theinorganic coating over the particles, the mineral oil is stripped fromthe particles. By way of example, the mineral oil may comprise siliconeoil. Lithium metal particles suspended in silicone oil are commerciallyavailable from Sigma-Aldrich of St. Louis, Mo.

Mineral oils such as silicon oil may be removed from the lithiumparticles by washing with a suitable cleaning solvent such astetrahydrofuran (THF) or methylene chloride. A vacuum filtration system,for example, may be used to wash the lithium particles. Due to thevolatility of the lithium, both the washing to remove an organicencapsulant and the act of contacting the lithium metal particles with acoating solution comprising a lithium metal salt to form the inorganicencapsulant can be carried out in a controlled atmosphere such as aglove box that is free or substantially free of oxygen and water. Priorto contacting the lithium metal particles with a coating solution, thewashed lithium particles can be dried. The washed particles can be driedby heating the particles to evaporate the solvent, e.g., up to a dryingtemperature of about 300° C.

To form the inorganic coating, a lithium salt is initially dissolved ina coating solvent to form a coating solution. Suitable solvents arecapable of dissolving the lithium salt. Example coating solvents includeTHF, n-methyl pyrrolidone (NMP), methylene chloride, or combinationsthereof.

After contacting the lithium particles with the coating solution, thecoating solvent can be removed to form a coating of the lithium saltover the particles. The solvent may be removed by evaporation, which mayeither occur naturally under environmental conditions of the preparationprocess or may be forced through various techniques including vacuumtechniques. For example, THF may be liberated through evaporation atroom temperature and with no vacuum. In a further example, NMP may beremoved by heating optionally with the application of vacuum. In variousembodiments, removal of the coating solvent may be performed at roomtemperature or by heating to a temperature of at most about 150° C.,e.g., about 30, 50, 75 or 300° C.

The thickness 332 of the coating 320 may be determined by controllingthe concentration of the lithium salt in the coating solution.Generally, a higher salt content in the solution will produce a thickercoating. A concentration in the coating solution of the lithium salt mayrange from about 0.1 to 4 molar, e.g., 0.1, 0.2, 0.5, 1, 2, 3 or 4molar. In embodiments, the coating solution comprises a saturatedsolution of the lithium salt.

In the resulting stabilized lithium composite particles, the lithiumsalt coating may comprise from about 1 to 50 wt. % of the total mass ofthe particles. For instance, the coating may comprise 1, 2, 5, 10, 20,30, 40 or 50 wt. % of the total mass. Together with the composition,this thickness of the coating is chosen to provide an effective barrierto the diffusion of air, oxygen and water.

EXAMPLES Example 1

Lithium-ion button cell capacitors were prepared with various ratios oflithium metal to graphite (anode material) in Samples 1-4.

To form the anode, a graphite slurry was prepared by mixing 100 ggraphite powder (Aldrich), 2 g carbon black (Cabot Corporation), 10 gpolyvinylidene fluoride (PVDF) (Alfa Aesar) and 190 gN-methyl-2-pyrrolidone (NMP) solvent (Sigma-Aldrich) to form a smoothslurry. The slurry was dispersed to a thickness of about 1 mm ontocopper foil using a doctor-blade. The graphite slurry-coated copper foilwas initially dried in a fume hood, vacuum dried in a 120° C. oven, andthen diced into 1.4 cm diameter electrodes. The typical thickness of thegraphite electrodes (graphite plus copper current collector) was about17 mm.

To form the cathode, activated carbon films were made by grindingactivated carbon powder and PTFE binder at a ratio of 90:10 by weight ata speed of 350 rpm to form a mixture that was rolled into a thin sheet.A typical thickness of the activated carbon layer was about 13 mm. Theactivated carbon was laminated onto aluminum foil (1 mil thickness)using carbon ink, and the resulting laminate was cut into 1.4 cmdiameter electrodes.

Lithium composite particles were provided having a lithium metal coreand a LiPF₆ layer encapsulating the core. Commercially-available lithiummetal particles in silicone oil were first washed and filtered with THFunder controlled atmosphere to remove the silicone oil. The particleswere dried and transferred to a dish containing a 2M coating solution ofLiPF₆ dissolved in THF. The solvent evaporates under ambient conditionsto produce stabilized, LiPF₆-coated lithium composite particles. Theamount and concentration of the coating solution was controlled toproduce composite particles where, upon drying, the weight ratio ofLiPF₆ (coating) to lithium metal (core) is about 20:80.

Sample 1. The lithium-ion capacitor included 68.0 mg graphite, 35.1 mgactivated carbon, and 6.8 mg lithium composite particles comprising 20%LiPF₆. The mass ratio of lithium composite particles to graphite was1:10.

Sample 2. The lithium-ion capacitor included 73.3 mg graphite, 34.4 mgactivated carbon, and 13.9 mg lithium composite particles comprising 20%LiPF₆. The mass ratio of lithium composite particles to graphite was1:5.27.

Sample 3. The lithium-ion capacitor included 66.6 mg graphite, 32.9 mgactivated carbon, and 22.1 mg lithium composite particles comprising 20%LiPF₆. The mass ratio of lithium composite particles to graphite was1:3.

Sample 4. The lithium-ion capacitor included 66.5 mg graphite, 35.2 mgactivated carbon, and 31.6 mg lithium composite particles comprising 20%LiPF₆. The mass ratio of lithium composite particles to graphite was1:2.1.

Lithium-ion capacitors were assembled using CR2032 button cell caseswith Al-clad (MTI Corporation) packages. The stacked electrode setincluded, in order, aluminum current collector/activated carboncathode/paper separator/lithium composite particles/graphiteanode/copper current collector. The electrolyte solution (˜0.3 g ofelectrolyte per button cell) was prepared using a 1M solution of LiPF₆in a 1:1 by volume mixture of ethylene carbonate (EC) and dimethylcarbonate (DMC) solvents.

The lithium-ion button cell capacitors were first tested using cyclicvoltammetry at a scan rate of 1 mV/s, with an operating voltage rangingfrom 2.2V to 3.8V. The capacities of lithium-ion capacitors weredetermined at constant current discharge (1 mA) after holding at 3.8Vfor 2 h. Power capabilities were measured at various currents afterholding at 3.8V for 1 h. Energy and power densities based the volume ofelectrodes and separators were calculated by integrating dischargecurves.

Summarized in Table 1 are energy and power density data for thelithium-ion capacitor Samples 1-4. It can be seen that higher energydensities were obtained for Samples 2 and 3. A dramatically lower energydensity was obtained for Sample 4. At the high loading ratio of Sample4, lithium metal may not be completely incorporated into the graphiteanode such that the unincorporated lithium may influence theinsertion-deinsertion of resulting in lower energy density. At a ratioof 1:10 (Sample 1), the energy density of the lithium-ion capacitor isslightly lower than that for Samples 2 and 3. However, the power densityof Sample 1 is slightly greater than the power density of Samples 2 and3.

Based on these data, lithium-ion capacitors with graphite negativeelectrodes can have a wide mass ratio range, e.g., from about 1:3 to1:10. FIG. 3 shows constant current (1 mA) discharge curves for Samples1, 3 and 4. The linear behavior of the data in FIG. 3 is consistent withgood capacitive behavior at higher voltages.

TABLE 1 Energy and Power density data for button cell electrodes withdifferent lithium:graphite mass ratios Sample Number 1 2 3 4 Lithiumcomposite particles:graphite 1:10 1:5.27 1:3 1:2.1 electrode weightratio Energy Density (Wh/l) 35.8 37.9 38.5 31.5 Power Density (W/l) 23.321.2 22.8 23.8 FIG. 3 reference number 410 N/A 408 406

Example 2

Lithium-ion button cell capacitors were prepared using hard carbon asthe anode material.

Sample 5. Ground phenolic resin was heated to 660° C. at a heating rateof 200° C./hour, held at 660° C. for 2 hours to carbonize the resin, andthen cooled to room temperature. The thermal cycle was performed underN₂ atmosphere with a gas flow rate of 6.18 l/min. The resulting carbonwas soaked in 37% HCl overnight and rinsed with deionized water toremove trace impurities. The sample was further soaked in a 29% NH₄OHaqueous solution overnight, followed by rinsing with deionized water.Carbon slurry was prepared using 100 g of the resulting carbon, 10 gpolyvinylidene fluoride (PVDF), 2 g carbon black and 190 g NMP solvent.

The lithium-ion capacitor of Sample 5 included 35 mg activated carbon(cathode), 48 mg of the above-described hard carbon (anode), and 13.2 mglithium composite particles comprising 20% LiPF₆.

Sample 6. Ground phenolic resin was heated to 1000° C. at a heating rateof 200° C./hour, held at 1000° C. for 2 hours to carbonize the resin,and then cooled to room temperature. The thermal cycle was performedunder N₂ atmosphere with a gas flow rate of 6.18 l/min. The resultingcarbon was soaked in 37% HCl overnight and rinsed with deionized waterto remove trace impurities. The sample was further soaked in a 29% NH₄OHaqueous solution overnight, followed by rinsing with deionized water.The purified carbon was heated at 1000° C. for 2 hours under N₂atmosphere. Carbon slurry was prepared using 42.5 g of the resultingcarbon, 5 g PVDF, 2.5 g carbon black and 120 ml NMP solvent.

The lithium-ion capacitor of Sample 6 included of 34 mg activated carbon(cathode), 43 mg of the above-described hard carbon (anode), and 13.4 mglithium composite particles comprising 20% LiPF₆.

Sample 7. Ground phenolic resin was heated to 660° C. at a heating rateof 200° C./hour, held at 660° C. for 2 hours to carbonize the resin, andthen cooled to room temperature. The thermal cycle was performed underN₂ atmosphere with a gas flow rate of 6.18 l/min. Carbon slurry wasprepared using 42.5 g of the carbon, 5 g PVDF, 2.5 g carbon black and150 ml NMP solvent.

The lithium-ion capacitor of Sample 7 included 36 mg activated carbon(cathode), 52 mg of the above-described hard carbon (anode), and 12.4 mglithium composite particles comprising 20% LiPF₆.

Sample 8. Wheat flour was heated to 1000° C. at a heating rate of 200°C./hour, held at 1000° C. for 2 hours to carbonize the flour, and thencooled down to room temperature. The thermal cycle was performed underN₂ atmosphere with a gas flow rate of 6.18 l/min. The carbon was soakedin 37% HCl overnight and rinsed with deionized water to remove traceimpurities. The sample was further soaked in 29% NH₄OH overnight,followed by rinsing with deionized water. The purified carbon wastreated at 1000° C. for 2 hours under N₂ atmosphere. Carbon slurry wasprepared using 42.5 g of the carbon sample, 5 g PVDF, 2.5 g carbon blackand 150 ml NMP solvent.

The lithium-ion capacitor of Sample 8 included 35 mg activated carbon(cathode), 42 mg of the above-described wheat flour-derived hard carbon(anode), and 14.2 mg lithium composite particles comprising 20% LiPF₆.

Performance of the hard carbon-based capacitors was measured by cyclicvoltammetry and constant current discharging. Energy and power densitiesof lithium-ion capacitors based on the volume of electrodes andseparators were calculated by an integration method. The reported energyand power densities are average results of two lithium-ion capacitors.

Table 2 summarizes average energy density and power density oflithium-ion capacitors as a function of discharge current. It can beseen that energy densities at 0.001 A for Samples 5, 6, and 8 range from34.8 Wh/l to 36.2 Wh/l, while the energy density for the lithium-ioncapacitor of Sample 7 is only 25.3 Wh/l.

TABLE 2 Energy and power density data for hard carbon-containing buttoncell electrodes as a function of discharge current. Sample 5 - resinSample 6 - resin Sample 7 - resin Sample 8 - Wheat flour 660° C., washed1000° C., washed 660° C., unwashed 1000° C., washed energy power energypower energy power energy power Current density density density densitydensity density density density (A) (Wh/l) (W/l) (Wh/l) (W/l) (Wh/l)(W/l) (Wh/l) (W/l) 0.001 35.9 22.8 36.2 23.6 25.3 22.5 34.8 25.0 0.00533.1 112.2 34.2 117.1 18.7 111.1 32.7 124.1 0.01 29.1 219.5 31.0 230.514.0 214.9 28.0 243.7 0.02 21.4 420.4 25.3 446.3 8.7 405.2 19.6 470.60.05 6.4 961.4 12.3 1033.6 1.9 928.7 6.0 1103.2 0.1 N/A N/A 2.8 1938.4N/A N/A 1.0 2043.4

Comparing the data for sample 5 with the data for sample 7 illustratesthe beneficial effects of a washing step. Comparing the data for sample5 with the data for sample 6 shows that increasing the processtemperature from 660° C. to 1000° C. increases the energy density forhigher discharge rates (e.g., 0.05 Amps, which is relevant to high powerapplications). Comparing the data for sample 6 with the data for sample8 shows that the synthetic resin-based carbon outperforms wheatflour-based carbon.

In an embodiment, the capacitor comprises a hard carbon-based anodewhere the hard carbon is derived from synthetic material (phenolicresin) that is carbonized at high temperature (˜1000° C.) andadditionally washed with ammonia and hydrochloric acid, i.e., sample 6.

FIG. 4 is a Ragone plot (power density versus energy density) and FIG. 5shows constant current discharge curves at 1 mA for Samples 5-8.Reference numerals identifying Samples 5-8 in FIGS. 4 and 5 aresummarized in Table 3.

TABLE 3 Summary of reference numerals. Sample Number 5 6 7 8 FIG. 4Reference Number 510 512 514 516 FIG. 5 Reference Number 610 612 614 616

FIG. 6 shows successive cyclic voltammograms of the lithium-ioncapacitor of Sample 6. A first cycle is shown as line 710 and a secondcycle is shown as line 712. It can be seen that the voltammograms have arectangle shapes, which indicates that the lithium-ion capacitor hadgood capacitive behavior.

Example 3

Lithium metal particles in silicone oil were first washed and filteredwith THF under controlled atmosphere to remove the silicone oil. Theparticles were dried and transferred to a dish containing a 2M coatingsolution of LiPF₆ dissolved in THF. The solvent evaporates under ambientconditions to produce stabilized, LiPF₆-coated lithium compositeparticles. The amount and concentration of the coating solution wascontrolled to produce composite particles where, upon drying, the weightratio of LiPF₆ (coating) to lithium metal (core) is about 20:80.

SEM micrographs of the coated particles are shown at high and lowmagnifications respectively in FIGS. 7A and 7B. Micrographs of theuncoated particles are not available due to their highly pyrophoricnature. In a 1 week evaluation, the coated particles remained stable atroom temperature in air. In a further evaluation, the coated particlesexhibited no reaction following overnight exposure in an oven at 150° C.

Example 4

The Example 3 experiment was repeated except using NMP as the solventfor LiPF₆. The sample was dried at 320° C. in a vacuum oven to removethe NMP. The resulting powder displayed comparable air stability toExample 3.

Example 5

The coating experiment was repeated with methylene chloride as solventfor LiPF₆ and results similar to examples 3 and 4 were obtained. An SEMmicrograph of the coated particles is shown in FIG. 8.

In one embodiment, the lithium-ion capacitor may have a power density ofat least about 1000 W/l (Watt/liter) such as at least about 500, 600,700, 800, 900, or 1000 W/l), and an energy density of at least about 10Wh/l (Watt-hour/liter) such as at least about 10, 20, 30, 40 or 50 Wh/l,at a current of about 0.05 A. In one embodiment, the lithium-ioncapacitor may have a power density of at least about 1033.6 W/l and anenergy density of at least about 12.3 Wh/l at a current of about 0.05 A.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “glass” includes examples having two or moresuch “glasses” unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, examples include from the one particular value and/or to theother particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It is also noted that recitations herein refer to a component being“configured” or “adapted to” function in a particular way. In thisrespect, such a component is “configured” or “adapted to” embody aparticular property, or function in a particular manner, where suchrecitations are structural recitations as opposed to recitations ofintended use. More specifically, the references herein to the manner inwhich a component is “configured” or “adapted to” denotes an existingphysical condition of the component and, as such, is to be taken as adefinite recitation of the structural characteristics of the component.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Sincemodifications, combinations, sub-combinations and variations of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and their equivalents.

We claim:
 1. A device comprising a cathode, an anode, a separatorpositioned between the cathode and the anode, lithium compositeparticles positioned between the anode and the separator, and anelectrolyte solution, wherein the electrolyte solution comprises anelectrolyte material dissolved in a solvent, and the lithium compositeparticles comprise a lithium metal core and a layer of a complex lithiumsalt encapsulating the core.
 2. The device according to claim 1, whereinthe cathode comprises activated carbon and the anode comprises graphiteor hard carbon.
 3. The device according to claim 1, wherein the anodecomprises graphite, carbon black, hard carbon, coke, or combinationsthereof.
 4. The device according to claim 1, wherein the lithiumcomposite particles are provided as a contiguous layer on aseparator-facing surface of the anode.
 5. The device according to claim1, wherein the electrolyte material comprises the complex lithium salt.6. The device according to claim 1, wherein the electrolyte materialconsists essentially of the complex lithium salt.
 7. The deviceaccording to claim 1, wherein a weight ratio of lithium compositeparticles to anode material ranges from about 1:3 to 1:10.
 8. A methodof producing a lithium-ion capacitor, comprising: providing a cathode,an anode, a separator, a solvent, and a lithium composite materialcomprising a lithium metal core and a layer of a complex lithium saltencapsulating the core; positioning the separator between the cathodeand the anode; positioning the lithium composite material between theanode and the separator to form an electrode set; and contacting theelectrode set with the solvent.
 9. The method of claim 8, wherein thesolvent dissolves the complex lithium salt.
 10. The method of claim 8,wherein the lithium composite particles have an average particle size ofabout 500 microns or less.
 11. The method of claim 8, wherein thecathode comprises activated carbon and the anode comprises graphite orhard carbon.
 12. The method of claim 8, wherein the lithium compositematerial is provided as a contiguous layer.
 13. The method of claim 8,wherein the lithium composite material is provided as a contiguous layeron a separator-facing surface of the anode.
 14. The method of claim 13,wherein hard carbon is produced by carbonizing a phenolic resin andwashing the carbonized phenolic resin with ammonia and hydrochloricacid.
 15. The method of claim 14, wherein the resin is carbonized at atemperature of at least about 1000° C.
 16. A lithium-ion capacitorcomprising a non-porous cathode, a non-porous anode, a separatorpositioned between the cathode and the anode, lithium compositeparticles positioned between the anode and the separator, and anelectrolyte solution, wherein the electrolyte solution comprises anelectrolyte material dissolved in a solvent, and the lithium compositeparticles comprise a lithium metal core and a layer of a saltencapsulating the core.